Lazy create StaticFileMiddleware statemachine (#9507)

src/Middleware/StaticFiles/ref/Microsoft.AspNetCore.StaticFiles.netcoreapp3.0.cs

@@ -103,7 +103,6 @@ namespace Microsoft.AspNetCore.StaticFiles
     public partial class StaticFileMiddleware
     {
         public StaticFileMiddleware(Microsoft.AspNetCore.Http.RequestDelegate next, Microsoft.AspNetCore.Hosting.IWebHostEnvironment hostingEnv, Microsoft.Extensions.Options.IOptions<Microsoft.AspNetCore.Builder.StaticFileOptions> options, Microsoft.Extensions.Logging.ILoggerFactory loggerFactory) { }
-        [System.Diagnostics.DebuggerStepThroughAttribute]
         public System.Threading.Tasks.Task Invoke(Microsoft.AspNetCore.Http.HttpContext context) { throw null; }
     }
     public partial class StaticFileResponseContext

src/Middleware/StaticFiles/src/StaticFileMiddleware.cs

@@ -68,7 +68,7 @@ namespace Microsoft.AspNetCore.StaticFiles
         /// </summary>
         /// <param name="context"></param>
         /// <returns></returns>
-        public async Task Invoke(HttpContext context)
+        public Task Invoke(HttpContext context)
         {
             var fileContext = new StaticFileContext(context, _options, _matchUrl, _logger, _fileProvider, _contentTypeProvider);
 
@@ -95,6 +95,14 @@ namespace Microsoft.AspNetCore.StaticFiles
             else
             {
                 // If we get here, we can try to serve the file
+                return ServeStaticFile(context, fileContext);
+            }
+
+            return _next(context);
+        }
+
+        private async Task ServeStaticFile(HttpContext context, StaticFileContext fileContext)
+        {
             fileContext.ComprehendRequestHeaders();
             switch (fileContext.GetPreconditionState())
             {
@@ -122,7 +130,8 @@ namespace Microsoft.AspNetCore.StaticFiles
                     {
                         context.Response.Clear();
                     }
-                        break;
+                    await _next(context);
+                    return;
                 case StaticFileContext.PreconditionState.NotModified:
                     _logger.FileNotModified(fileContext.SubPath);
                     await fileContext.SendStatusAsync(Constants.Status304NotModified);
@@ -139,8 +148,5 @@ namespace Microsoft.AspNetCore.StaticFiles
                     throw exception;
             }
         }
-
-            await _next(context);
-        }
     }
 }